1. Field of the Invention
The invention pertains to a method for the manufacture, by coprecipitation, of zinc oxide based powders intended especially for making varistors.
2. Description of the Prior Art
Various prior art methods exist for the manufacture of zinc oxide based powders used to make varistors. One of these methods consists in mixing defined proportions of powdered oxides or salts of the elements (zinc, bismuth, antimony, cobalt, manganese, etc.) of which the varistor will be composed. The mix is then homogenized as intimately as possible, then compressed and fritted. This mechanical method does not give very fine-grained dopes as compared with zinc oxide and the homogeneity of the powder is affected by it. Chemical methods give more homogeneous mixes on the microscopic scale. By precipitation or separate partial coprecipitation, precursor salts dissolved in various solutions can be used to give hydroxides which, when further mixed and subjected to heat treatment, give powders. The homogeneity of the powders may be greater than that of powders obtained by the direct mechanical mixing of oxides. However, the electrical characteristics of the varistors prepared with these powders show that these methods are still inadequate in terms of the homogeneity and reproducibility of the products obtained. A high level of homogeneity in the powders would give a better distribution of the potential barriers at grain boundaries as well as greater reproducibility.
The invention proposes to achieve these aims by the simultaneous precipitation of all the elements which are to form the powder, with a composition which is defined at any instant of the coprecipitation process. The method proposed includes the addition of elements with a given overall composition to a pre-saturated solution of all these elements.
This pre-saturated solution is buffered at a specified pH and the total buffer concentration is monitored. The pH is calculated and kept constant throughout the coprecipitation. This can be obtained by keeping the total buffer concentration of the saturated solution constant throughout the coprecipitation operation, and also keeping the total volume of this solution constant.